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Eventually, this definition too was found to be inadequate for precise time measurements, so in 1967, the [[SI second]] was again redefined as 9,192,631,770 periods of the radiation emitted by a [[caesium]]-133 atom in the transition between the two hyperfine levels of its ground state.<ref name="USNO">{{cite web|title=Leap Seconds|publisher=Time Service Department, [[United States Naval Observatory]]|url=https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/Leap-Seconds/|access-date=19 November 2022}}</ref> That value agreed to 1 part in 10<sup>10</sup> with the astronomical (ephemeris) second then in use.<ref>[[William Markowitz|Wm Markowitz]] (1988) 'Comparisons of ET (Solar), ET (Lunar), UT and TDT', in (eds.) A K Babcock & G A Wilkins, 'The Earth's Rotation and Reference Frames for Geodesy and Geophysics', IAU Symposia #128 (1988), at pp 413–418.</ref> It was also close{{quantify|date=January 2022}} to {{frac|1|86,400}} of the mean solar day as averaged between years 1750 and 1892.
However, for the past several centuries, the length of the mean solar day has been increasing by about 1.4–1.7
When the [[Coordinated Universal Time]] (UTC) standard was instituted in 1960, based on atomic clocks, it was felt necessary to maintain agreement with UT, which, until then, had been the reference for broadcast time services. From 1960 to 1971, the rate of UTC atomic clocks was offset from a pure atomic time scale by the [[International Time Bureau|BIH]] to remain synchronized with [[UT2]], a practice known as the "rubber second".<ref>{{cite book|title=From Sundials To Atomic Clocks: Understanding Time and Frequency |first1=James |last1=Jespersen |first2=Jane |last2=Fitz-Randolph |publisher=[[National Institute of Standards and Technology]] |url=https://tf.nist.gov/general/pdf/1796.pdf |page=109 |year=1999}}</ref> The rate of UTC was decided at the start of each year, and was offset from the rate of atomic time by −150 parts per 10{{sup|10}} for 1960–1962, by −130 parts per 10{{sup|10}} for 1962–63, by −150 parts per 10{{sup|10}} again for 1964–65, and by −300 parts per 10{{sup|10}} for 1966–1971.<ref name=NBS140>{{citation |editor-last=Blair |editor-first=Byron E. |title=NBS Monograph 140: Time and Frequency: Theory and Fundamentals |url=https://nvlpubs.nist.gov/nistpubs/Legacy/MONO/nbsmonograph140.pdf |date=May 1974 |page=8}}</ref> Alongside the shift in rate, an occasional 0.1 s step (0.05 s before 1963) was needed. This predominantly frequency-shifted rate of UTC was broadcast by [[Time from NPL (MSF)|MSF]], [[WWV (radio station)|WWV]], and [[CHU (radio station)|CHU]] among other time stations. In 1966, the [[ITU-R#CCIR|CCIR]] approved "stepped atomic time" (SAT), which adjusted atomic time with more frequent 0.2 s adjustments to keep it within 0.1 s of UT2, because it had no rate adjustments.<ref>{{cite book|title=Time: From Earth Rotation to Atomic Physics|edition=second|first1=Dennis D.|last1=McCarthy|first2=P. Kenneth|last2=Seidelmann|quote=For provisional limited use, the CCIR in 1966 approved "Stepped Atomic Time," which used the atomic second with frequent 200 ms adjustments made in order to be within 0.1 s of UT2.}}</ref> SAT was broadcast by [[WWVB]] among other time stations.<ref name=NBS140/>
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